Image forming apparatus having grid electrode with opening and non-opening portions

Abstract

An image forming apparatus includes a corona charger including a discharging wire and a grid electrode for charging a photosensitive drum at a charging position and an exposure device for exposing the drum charged by the charger at an exposure position which is at a downstream side of the charger (in a rotational direction of the drum) at the charging position. The grid electrode includes a first portion having openings and a second portion having only a non-opening portion in a charging region for charging the drum in a longitudinal direction of the charger, and the second portion includes a downstream side peripheral portion at a downstream end of the grid electrode with respect to the rotational direction. In a cross-section orthogonal to the longitudinal direction and including the charging region, the downstream side peripheral portion is intersected by a straight line passing through the exposure position and the wire.

What is claimed is: 
     
       1. An image forming apparatus comprising:
 a rotatable photosensitive member; 
 a corona charger including a discharging wire and a plate-like grid electrode and being configured to charge a surface of said photosensitive member at a charging position; and 
 an exposure device configured to expose the surface of said photosensitive member charged by said corona charger at an exposure position, which is at a downstream side of said corona charger in a rotational direction of said photosensitive member at the charging position, to form an electrostatic image, 
 wherein said grid electrode includes a first portion having a plurality of openings and a second portion provided adjacent to said first portion with respect to the rotational direction, said second portion including a downstream side peripheral portion not provided with an opening in a charging region in a longitudinal direction of said corona charger, and 
 wherein in a cross-section orthogonal to the longitudinal direction and including the charging region, the downstream side peripheral portion extends from a downstream end portion of said plate-like grid electrode at least to a position crossing with a straight line A passing through the exposure position and said discharging wire. 
 
     
     
       2. An image forming apparatus according to  claim 1 , wherein said corona charger includes upstream and downstream shield portions sandwiching said discharging wire and being substantially perpendicular to said grid electrode, and a gap is provided in the cross-section between said downstream shield portion and a downstream side end portion of said downstream side peripheral portion with respect to the rotational direction. 
     
     
       3. An image forming apparatus according to  claim 1 , wherein said grid electrode includes an upstream side peripheral portion at an upstream end of said grid electrode with respect to the rotational direction, and a length of the downstream side peripheral portion measured in the rotational direction is longer than a length of the upstream side peripheral portion measured in the rotational direction. 
     
     
       4. An image forming apparatus according to  claim 1 , further comprising a fan capable of generating a flow of air in the direction from a discharging wire side surface of said grid electrode toward a photosensitive member side surface of said grid electrode. 
     
     
       5. An image forming apparatus according to  claim 1 , wherein the downstream side peripheral portion extends toward the downstream side at least to a tangential line B of said photosensitive member passing through said discharging wire at the downstream side in the rotational direction. 
     
     
       6. An image forming apparatus comprising:
 a rotatable photosensitive member; 
 a corona charger including a discharging wire and upstream and downstream shield portions sandwiching said discharging wire, and being configured to charge a surface of said photosensitive member at a charging position; 
 an exposure device configured to expose the surface of said photosensitive member charged by said corona charger at an exposure position, which is at a downstream side of said corona charger in a rotational direction of said photosensitive member at the charging position, to form an electrostatic image; and 
 a shielding member configured to shield said photosensitive member from said discharging wire, said shielding member extending from the downstream shield portion at least to a straight line A passing through the exposure position and said discharging wire in a cross-section orthogonal to the longitudinal direction and including a charging region. 
 
     
     
       7. An image forming apparatus according to  claim 6 , wherein a plate-like grid electrode is provided between said shielding member and said discharging wire.